You are honestly "not seeing the issue" with the idea to mandate that people are free to dwell permanently on any land to which they have rights to, seriously?
Lets make it a simple scenario for the sake of the message;
I am Mr Farmer, i own 1000 acres of green belt land around Gerrards Cross and Watford. Suddenly they allow me to park caravans anywhere and let people live in them, permanently.
Do I:
a) Continue to grow wheat
b) Sublet my 1000 acres to 10000 individuals, and create a huge trailer park infilling between all of the local villages, with great access to the M25 and the tube network into London? Thus creating a gigantic slum/trailer park.
Miles and miles of this, as seen elsewhere in the world: